Page 315 - Basic Principles of Textile Coloration

304 DYEING CELLULOSIC FIBRES WITH DIRECT DYES

from being saturated with dye, so the dye only adsorbs onto a very small part of the
internal surface.

Covering neps of immature cotton fibres (Section 5.2.3) is always a concern in
cotton dyeing. With dyes of low substantivity, neps usually absorb less dye and
appear as paler spots on the fabric. Since immature fibres give much greater rates
of dye desorption, the paler dyed neps may not appear until after washing. Better
coverage can be achieved using dyes of high substantivity and can be much
improved by mercerisation before dyeing, since this swells the immature fibres
(Section 5.4.6).

Mercerised cotton will absorb more dye and be darker in colour than normal
cotton dyed in the same bath. It is more accessible than regular cotton and its
lower negative surface potential decreases repulsion of dye anions. It will appear
darker in colour even for the same amount of dye in the fibre because of a higher
degree of internal light scattering that results in greater light absorption. This
effect is, however, dependent on the dye being used and the conditions of
mercerisation.

Viscose fibres and filaments are available in a number of variants with different
dyeing characteristics. They have different internal structures, skin-to-core ratios,
porosities and degrees of fibrillation. These different forms therefore have different
dyeing rates, equilibrium dye absorptions and colour yields. Direct dyes have
higher substantivity for regular viscose than for cotton because viscose is less
crystalline and oriented so it has a much greater internal surface. The fastness to
light of dyeings of direct dyes on viscose is up to one fastness grade better than on
cotton. The fastness to washing on viscose, however, is often less than on cotton
because of its higher accessibility. The more highly oriented viscose types give
better wet fastness.

The physical and chemical properties and the dyeing behaviour of the various
cellulosic fibres are therefore quite different. About all they have in common, is
the cellulose that constitutes them.

14.7 THE ORIGINS OF SUBSTANTIVITY FOR CELLULOSE

The dyeing of cotton with direct dyes is completely reversible. For dyeings at
equilibrium, the Freundlich isotherm usually applies (Section 11.1.1) but some
dyeings with direct dyes give a better correlation with the Langmuir isotherm. The
former isotherm usually indicates that adsorption is non-specific and the latter